Hybrid electric vehicles having an internal combustion engine combined with a motor-generator and an electrical energy storage system have been the focus of considerable attention in the automotive field, particularly in the field of passenger vehicles. Development of hybrid electric vehicle systems has only recently begun to attract significant interest in commercial and off-road vehicles, e.g., trucks and busses in Vehicle Classes 2-8, in earth-moving equipment and railroad applications, and in stationary internal combustion engine-powered installations.
Hybrid electric technologies offer numerous advantages, including improvements in fuel efficiency, reduction in internal combustion engine emissions and vehicle noise to help meet government regulatory requirements, improved vehicle performance and lower fleet operating costs. These advantages are obtained in significant part by hybrid electric systems' ability to recapture energy which would otherwise be wasted (such as mechanical energy from braking that would otherwise be dissipated as thermal energy to the environment) and return of the captured energy at another time when needed, such as powering vehicle components in lieu of using the internal combustion engine as the source of power or assisting in vehicle propulsion.
Typically, hybrid electric vehicle motor-generators have been arranged either independently of the internal combustions engine (for example, using separate electric motors to power and recover energy from front wheels while the engine provides propulsion power to the rear wheels), or have been coupled to the engine, for example being integrated into the “rear” of the engine (i.e., the end at which the engine's flywheel is located) or between the engine and the driveline to the wheels. This “behind the engine” position permits the motor-generator equipment to deliver torque directly to the vehicle's driveline and wheels, and to be directly driven by the driveline, for example, during regenerative braking events. Examples of the latter include flywheel-type motor-generators in which a conventional engine's flywheel is modified to serve as a motor-generator rotor and a concentrically-mounted stator is located around the flywheel, and separate electric motors arranged between the engine and the drive wheels, such as the so-called “two mode hybrid” transmission offered by General Motors in the 2009 GMC Silverado light-duty pickup in which the transmission accommodated two electric motors for vehicle propulsion and electric energy generation.
Another form of adding a motor-generator to an internal combustion engine is the use of so-called starter-generators. This approach directly couples an electric motor to an engine to serve both as an electric generator (a function traditionally performed by a conventional belt-driven alternator) and as an engine starter, thereby reducing the weight and cost of duplicate alternator and starter electric motors. Such starter-generator installations are particularly useful in so-called engine stop-start systems which turn off the engine during periods when the vehicle is stopped to save fuel and reduce idling emissions. Starter-generators have been located behind the engine (for example, an appropriately engineered flywheel motor-generator may also be used as a starter), as well as being mounted at the front end of an engine where the starter-generator can drive a belt directly coupled to the engine crankshaft. An example of the latter system the “belt alternator starter” system that was offered by General Motors as an option in the 2007 Saturn Vue sport-utility vehicle. These systems are very difficult to adapt to large engines, such as commercial vehicle diesel engines, because the electric motor must be larger to deal with the much higher torque demands of these heavy-duty engines, such as starting and operating various components (for example, an engine cooling fan can demand upwards of 50 KW of power, a load that requires a large amount of torque to drive the fan belt). Further, the belt drive in such an enlarged system would need to have the capacity to transfer the large levels of torque, something that may not be possible, or at least practical, because thicker and broader drive belts and pulleys sufficient to handle the torque demands may be so much larger and heavier than their automotive counterparts that they are weight, size and/or cost prohibitive.